In prior arts, coded bits are generally added to improve the quality of speech/audio signals encoded by traditional encoding methods, such as the pulse code modulation (PCM) method and the adaptive differential pulse code modulation (ADPCM) method.
In the process of implementing the present invention, the inventor finds at least the following problems in the prior arts:
After coded bits are added, if a decoder supports only the decoding of encoded signals of a low bit rate, or if the decoder receives only the encoded signals of the low bit rate due to insufficient network bandwidth and poor transmission quality, the decoder can only decode the encoded signals of the low bit rate, resulting in poor quality of decoded signals. For example, if the encoder and decoder use the traditional ADPCM encoding and decoding method, the encoded signals of a low bit rate decoded by the decoder are even worse than the decoded encoded signals that are generated without adding extra bits.